RsaL is a self‐regulatory switch that controls alternative biosynthesis of two AHL‐type quorum sensing signals in Pseudomonas aeruginosa PA1201

Pseudomonas aeruginosa is a ubiquitous and metabolically versatile microorganism naturally found in soil and water. It is also an opportunistic pathogen in plants, insects, animals, and humans. In response to increasing cell density, P. aeruginosa uses two acyl‐homoserine lactone (AHL) quorum‐sensing (QS) signals (i.e., N‐3‐oxo‐dodecanoyl homoserine lactone [3‐oxo‐C12‐HSL] and N‐butanoyl‐homoserine lactone [C4‐HSL]), which regulate the expression of hundreds of genes. However, how the biosynthesis of these two QS signals is coordinated remains unknown. We studied the regulation of these two QS signals in the rhizosphere strain PA1201. PA1201 sequentially produced 3‐oxo‐C12‐HSL and C4‐HSL at the early and late growth stages, respectively. The highest 3‐oxo‐C12‐HSL‐dependent elastase activity was observed at the early stage, while the highest C4‐HSL‐dependent rhamnolipid production was observed at the late stage. The atypical regulator RsaL played a pivotal role in coordinating 3‐oxo‐C12‐HSL and C4‐HSL biosynthesis and QS‐associated virulence. RsaL repressed lasI transcription by binding the –10 and –35 boxes of the lasI promoter. In contrast, RsaL activated rhlI transcription by binding the region encoding the 5′‐untranslated region of the rhlI mRNA. Further, RsaL repressed its own expression by binding a nucleotide motif located in the –35 box of the rsaL promoter. Thus, RsaL acts as a molecular switch that coordinates the sequential biosynthesis of AHL QS signals and differential virulence in PA1201. Finally, C4‐HSL activation by RsaL was independent of the Las and Pseudomonas quinolone signal (PQS) QS signaling systems. Therefore, we propose a new model of the QS regulatory network in PA1201, in which RsaL represents a superior player acting at the top of the hierarchy.